One of the major principles of Einstein's theory of relativity
is that space-time is curved around a massive object such as
a black hole.
In a geometry that is curved (as opposed to flat or
Euclidean), objects and photons followed curved paths rather
than straight lines, and the sum of the angles of a triangle
is not equal to 180o.
According to Einstein's theory, the warping of space-time
around the Sun causes the
observed advance of the perihelion of Mercury
and the bending of starlight as it passes close
to the Sun.